1. Field of the Invention
The present invention relates to a photometric device which detects the illuminance or the amount of light.
2. Description of the Related Art
A photoelectric conversion element such as a photodiode generates photocurrent corresponding to the illuminance of light with which the photoelectric conversion element is irradiated. The illuminance of light can be measured by detecting the amount of the photocurrent. An optical sensor which amplifies photocurrent generated in a photodiode to be output as a current signal (an analog signal) by using the above mechanism has been put into practical use.
Further, electronic devices provided with an optical sensor, such as a television receiver and a mobile phone, have been commercialized. Such products have a function of adjusting the luminance of a display screen by detecting the illuminance of usage environment by the optical sensor. Therefore, in an electronic device which is controlled and operated by a digital signal, an output from the optical sensor is required to be a digital signal.
For example, Patent Document 1 discloses an optical sensor in which electric charge stored in a capacitor with photocurrent is discharged with constant current and the amount of electric charge generated in a photodiode is measured using the discharging period. The optical sensor in Patent Document 1 measures the discharging period by counting the number of pulses of a clock signal and generates a digital signal including the count value as data.